In the event of a fault e.g. in the event of a cable interruption, laser radiation in an optical transport network (OTN), which at a certain intensity can result in damage to eyesight, may be released. ITU-T G.664 “Optical Safety Procedures and Requirements for Optical Transport Systems”, June 1999, specifies in respect of SDH (synchronous digital hierarchy) networks and optical point-to-point connections methods for disconnecting lasers or for reducing the laser radiation being released. ITU-T G.664 also specifies methods which enable the connection of the transmission route at the time of startup or after clearing a fault. Laser disconnections are, however, carried out not only in cases of faults but also e.g. where a network is reconfigured.
ITU-T G.664 describes both a manual and an automatic mode for connecting lasers. In the case of the manual method, the laser is connected manually. For safety reasons, the connection duration is only 2 seconds (up to a maximum of 90 seconds in the case of manual tests). If the transmission route is intact, a receiver at one end of a transmission route detects the laser light and then also connects a laser there which feeds the light in the return direction to a transmitter location disposed at the start of the transmission route. The transmission route is thus operational. In the case of the automatic method, the disconnected laser is connected at intervals of 100-300 seconds for a duration of 2 seconds. If the transmission route is intact, the receiver at the end of the transmission route detects light and connects the laser there, which feeds the light in the return direction to a transmitter location disposed at the start of the transmission route. As a result of the pauses between the laser pulses, the startup of the transmission route may in this case be delayed by a maximum of 300 seconds.
In a transparent optical network, optical signals are switched without optical-electrical conversion. For example, a known switching technology is based on the use of MEMS (micro-electrical-mechanical system). This means that where an optical connection between a laser at the transmitter end and a receiver disposed at the end of the transmission route is not switched, the receiver detects no light and consequently, as in the event of a fault, disconnects a laser at the transmitter end by means of radiation in the return direction. In the case of optical connections between transmitter and receiver not being switched, the lasers are thus automatically deactivated. If an optical connection is then established as per automatic mode in accordance with ITU-T G.664, it can take up to a maximum of 300 seconds until the lasers are connected in the case of this automatic method. Such a long delay time cannot be tolerated.
FIG. 1 shows by way of example a network for transmitting optical signals in which the disconnection and connection of lasers could be applied as per the manual and automatic modes in accordance with ITU-T G.664. An optically transparent switching range TS of the network has several transparent network nodes without optical-electrical converters—here four elements ONK1, ONE2, ONK3, ONK4—which are connected to one another by means of optical fibers LWLij (i<j=1, 2, 3, 4) and enable differing transparent switchings of the transmitted optical signals. Outside the transparent switching range, a network node NK1 at the transmitter end, comprising an electrical-optical converter, i.e. comprising a laser source, is connected to the first transparent network node ONK1. A network node NK2 at the receiver end, comprising an electrical-optical converter, is connected to the third transparent network node ONK3. By this means, optical signals, for example, can be transmitted from the network node NK1 at the transmitter end via the transparent network nodes ONK1, ONK3 having an intermediately connected optical fiber LWL 13 to the network node NK2 at the receiver end. A transponder TL2, TL1 at the transmitter end and/or receiver end, comprising laser sources and optical receivers, can now be connected respectively to one of the second and fourth transparent network nodes ONK2, ONK4. If a fault occurs e.g. in the optical fiber LWL 13, in automatic mode for monitoring for laser safety purposes in accordance with ITU-T G.664, the lasers in the network nodes NK1, NK2 and in the transponders TL1, TL2 remain disconnected for several hundred seconds before being reconnected, although a rerouting of the optical signals via optical fibers LWLij other than LWL13, as well as a bar on switching to the faulty optical fiber LWL13, would be possible immediately or in a few seconds.